The long term objective of this proposal is to develop anti-cancer therapeutics which stabilize the inactive conformation of two protein kinases. Most kinase inhibitors compete with ATP for binding to the structurally conserved active site and thus despite high potency in vitro, show modest potency in vivo and limited specificity. This proposal outlines the development of a functionally novel class of kinase inhibitors expected to exhibit greater efficacy and reduced toxicity relative to those currently being developed. A newly emerging "covalent tethering" technology will be used to identify drug-like fragments that bind to a protein target, followed by their assembly through chemical linkage into potent inhibitors. This efficient "screen-then-link" process offers a greater survey of chemical diversity space than typical lead discovery processes. Two prototypic kinases, constrained in their inactive conformation, will be used in these experiments. Extenders will be synthesized and used to modify these kinases, which will subsequently be used to screen our library of >12,000 custom-made sulfhydryl containing drug-like fragments. Promising hit compounds will serve as the starting point for medicinal chemistry lead development. Once optimized, this approach will be applicable to the development of a large number of protein kinase inhibitors with significant anti-cancer therapeutic value.